The kiax Makefile ( work/kiax-0.8.51-src/src/Makefile ) has a target
install_icons which is supposed to copy the icon files to .destdir.
I am not sure whether this (apparently non standard) target is getting
invoked or not.
How to add invocation of such additional target in pkgsrc?

You should dig why the target is not invoked for you instead.
--
OBATA Akio / obache%NetBSD.org@localhost